1. Field of the Invention
The present invention relates to an optical pickup apparatus used for an optical type recording and reproducing apparatus, and more particularly to an optical pickup apparatus structured in a thin type.
2. Description of the Related Art
In recent years, it is developed to make a portable type personal computer thin, compact and light. Further, a storage capacity of a memory apparatus used in the portable type personal computer tends to be significantly increased, so that an external memory apparatus having a large amount of capacity is necessary. Optical type recording mediums such as a compact disc (CD, having a low density) and a digital versatile disc (DVD, having a high density) are used for the external memory apparatus of the portable type personal computer since they have a great amount of capacity and can be easily treated. As a result, the optical type recording and reproducing apparatus is required to be made thin, compact and light. In particular, it becomes a key to making the optical type recording and reproducing apparatus thin to structure an optical pickup apparatus for optically accessing the medium in a thin type.
Hereinafter, a description will be given of a conventional optical pickup apparatus with reference to the accompanying drawings. FIG. 4 is a perspective view of a conventional optical pickup apparatus, FIG. 5 is an exploded perspective view and FIG. 6 is an exploded perspective view of a portion of a magnetic circuit in FIG. 4.
As shown in FIGS. 4 to 6, a suspension base 2 is fixed to a carriage 1. Further, an end of each of four suspension wires 3 is fixed to the suspension base 2. A lens holder 7 holds an object lens 4, a focus coil 5 and a track coil 6. Further, the other end of each of the suspension wires 3 is fixed to the lens holder 7 so as to support the lens holder 7 by the suspension wires 3 in a cantilever manner.
A magnet 8 is fixed to a yoke 9 so as to fix the yoke 9 to the carriage 1. In this case, the focus coil 5 and the track coil 6 are arranged so that they are within a magnetic field formed by opposing two magnets 8 to each other and cross a magnetic flux. Then, it is possible to drive the lens holder 7 in a focusing direction and a tracking direction by energizing the focus coil 5 and the track coil 6. Finally, a closed magnetic circuit is constituted by mounting a cap yoke 10 to an opening portion of the yoke 9.
FIG. 7 is a schematic view which explains a shape of a portion of a magnetic circuit in FIG. 6. In FIGS. 6 and 7, reference symbol w denotes a value of a horizontal width of the magnet 8 and reference symbol h denotes a value of a height in a standing direction. For example, the thinnest conventional optical pickup is formed in such a manner as to have a dimension of w=3.4 mm and h=4.2 mm, and structured so as to have a relation of w&lt;h. This is because a vertically-long shape is formed for securing a large number of turns in the focus coil 5 and the track coil 6 and simultaneously for securing a sufficient focus stroke.
Further, at this time, a number of turns in the 10 focus coil 5 is expressed by a reference symbol n, a current flowing in the focus coil 5 is expressed by a reference symbol i, a magnetic flux density of a magnetic field (that is, a portion of both coils) generated by the opposing magnets 8 is expressed by a reference symbol B, and an effective length of the focus coil 5 crossing the magnetic flux is expressed by a reference symbol L, respectively. Further, since the portion crossing the magnetic flux is effective, the effective length of the focus coil 5 is the same as the value w of the horizontal width of the magnet 8. In this case, in an embodiment of the conventional optical pickup apparatus, n=141 turns, a finishing height of the focus coil 5 is 2.4 mm, and a thickness of the yoke 9 is 0.8 mm. Accordingly, a total height ht of the portion of the magnetic circuit (refer to FIG. 4) obtained by adding the thickness of the yoke 9 and the thickness of the cap yoke 10 to the value h of the height in the standing direction reaches 6.2 mm, and a total thickness t of a pickup module (refer to FIG. 4) including the carriage 1 requires 7.5 mm.
Reference numeral 20 denotes a line of magnetic flux generated by the magnet 8, which corresponds to a closed line circulating a rotation along the magnetic circuit. For reasons of explanation, it is supposed that the number of the lines of magnetic flux is six. Further, it is supposed that the number corresponds to a largest number of the lines of magnetic flux which can pass through the magnetic circuit on the basis of the strength of the magnet 8 and the cross sectional area of the yoke 9.
With respect to the conventional optical pickup apparatus structured in the manner mentioned above, an operation thereof will be described below. FIG. 8 is a schematic view which explains an operation of the portion of the magnetic circuit shown in FIG. 4. In FIG. 8, it is supposed that magnetic poles (N, S) of the magnet 8 are arranged in such a manner as shown in FIG. 8. For example, as shown by an arrow If, when energizing the focus coil 5, the focus coil 5 receives a force shown by an arrow Ff in accordance with Fleming's left-had rule, and the lens holder 7 moves upward in the drawing. Further, as shown by an arrow It, when energizing the track coil 6, the track coil 6 receives a force shown by an arrow Ft in accordance with Fleming's left-hand rule in the same manner, and the lens holder 7 moves to the leftward in the drawing. An electromagnetic force generated in both coils at this time and applied thereto is expressed by the following formula: EQU Electromagnetic force=niBL (1)
in which as mentioned above, reference symbol n denotes a number of turns of the coil, reference symbol i denotes a current flowing in the coil, reference symbol B denotes a density of the magnetic flux in both coil portions generated by the magnet 8, and reference symbol L denotes an effective length of the coil horizontally crossing the magnetic flux.
The conventional optical pickup apparatus as mentioned above is structured such that the size w in the direction of the horizontal width is made smaller than the height h in the standing direction of the yoke 9 for securing a stroke in the focusing direction. The height of the coil winding the coil is reduced when simply making the portion of the magnetic circuit thin in accordance with the requirement for making the optical pickup apparatus thin, so that the number of turns n of the coil shown in the formula (1) becomes small, and it is impossible to maintain a necessary electromagnetic force.
Then, for compensating for reduction of the number of turns n of the coil, it can be considered to strengthen the magnet 8 so as to increase the density B of the flux and to remove the cap yoke 10. However, the strengthening of the magnet 8 tends to be consumed by a magnetic saturation of the yoke 9. Further, when removing the cap yoke 10, the magnetic saturation of the yoke 9 is further easily generated. Accordingly, there is a limit of increasing the density B of the magnetic flux. As mentioned above, in the conventional optical pickup apparatus, there has been a limit of making it thin.
The present invention is made for solving the problems as mentioned above, and an object of the present invention is to provide an optical pickup apparatus having a structure suitable for making thin, compact and light.